You'd be very hard-pressed to find a farmer who doesn't supplement chickens with a little grain. If the majority of time is spent on pasture, that's what you want to hear. If they supplement with non-GMO grains, giddyup.

Jo Robinson at eatwild.com has an article about cows finished for a month on grains, for flavor and marbling, losing much of their healthful nutrient profile. But this farm states:

Occasionally we will "treat" the cattle to some non-GMO organic grain a few weeks prior to butcher. It is never considered part of their diet.

If they're just giving the cows some occasional grains as a treat, that's no big deal at all.

It certainly looks like they're committed to doing it right, raising animals on pasture/grass, no antibiotics or hormones, humane practices, etc. I always say one of the best things anyone can do is to visit their farms and talk to their farmers. But based on what you've shared they look like a good place to patronize.